For some time now a variety of techniques have been used to fabricate force sensors which provide an indication of the force applied between a pair of mating surfaces. These techniques have included the utilization of thin layers of semi-conductive materials disposed between the surfaces which respond to applied loads and which, when properly provided with conductors and associated circuitry, facilitate the display of indications of applied loads.
Early versions of products employing some of those features include those disclosed in U.S. Pat. Nos. 3,806,471 and 4,489,302. The common characteristic of those products is that they employ a body of semi-conductive material which, when stressed by the application of a load, will increase in conductivity. That increase in conductivity, which tends to increase as a function of the applied load, may then be used to provide a measurable output which varies, within limits, as a function of the applied load. Force sensing systems employing semi-conductive materials and based upon these principles are additionally shown by U.S. Pat. No. 4,856,993.
Typically semi-conductive layers used in force sensors must have certain characteristics to be sufficiently electrically conductive to be effective. Thus such layers must have electrically conductive areas which are close enough together to allow conduction under load. Under load the conductive areas must contact each other or the distances between them must be so small that electrons can flow from one conductive area to the next. The concentration of conductive areas must be large enough to provide a conductive path through the layer. The conductivity through the layer must be sufficient, under load, to provide a reliable and consistent range of different resistances (or conductances) to be able to distinguish among a range of applied loads. Typically the application of a load increases the capacity of the layer to allow electron transfer. Further, the conductivity changes should be reversible to the extent that the layer and surfaces on which the layer is applied permit restoration of the characteristics of the layer which are altered as load is applied. The pressure-sensitive, load responsive characteristics may be at the surface of the layer or internally thereof, or both.
A variety of intrinsically semi-conductive materials have been used to provide force sensors of this type. Such materials include particulate molybdenum disulfide, and ferrous and ferric oxide, among others. Such materials are disclosed in the patents referred to above, as well as in U.S. Pat. No. 5,296,837.
In addition to the use of semi-conductive systems to produce force sensing transducers, particulate conductive materials have also been used to produce force sensing transducers, as exemplified by the disclosure of U.S. Pat. No. 5,302,936. This patent and U.S. Pat. No. 5,296,837 both disclose the use of carbon as a conductive material in force sensing inks. The latter patent uses stannous oxide as a semi-conductive material in combination with carbon.
In more recent times, as shown by the prior art referred to above, semi-conductive, pressure-sensitive transducers have been made by depositing semi-conductive material, as in the form of an "ink" deposited by spraying or by a silk screening process, to form a thin layer or layers between a pair of electrodes. Typically, the electrodes are disposed on thin, flexible plastic sheets and have leads to a remote region in which the flow of an applied current may be sensed and measured. In such sensors, the electrodes and dried ink residue form a sandwich which acts as a force transducer, and which will provide a variable resistance (or conductance) which is related in a predetermined manner, to applied loads.
The prior art also teaches the use of blends of semi-conductive particles and conductive particles to provide a variably conductive force transducer. In particular, the prior art teaches the use of molybdenum disulfide as a semi-conductor blended with graphite or finely divided conductive carbon (such as acetylene black). The conductivity of inks based on these materials may be varied by the concentrations or ratios of the conductive and semi-conductive particles, frequently by blending a highly conductive ink with a less conductive ink. Polyester is the binder frequently used to bind the particles in these inks to a substrate on which a dried layer of the deposited materials is disposed. The resistance of the dried layer varies with load; hence these inks are referred to as being pressure-sensitive or force-sensitive.
These prior art inks have a number of shortcomings. For example, conventional binders, such as polyester binders, limit the useful application temperatures to a range of from up to 120.degree. to no more than about 150.degree. F. Above that temperature range, binders in confronting semi-conductive layers tend to bond to each other. Further, conductive carbon black when used as a pigment in resistive inks is very difficult to disperse uniformly and tends to agglomerate after dispersion. In addition its surface reactivity and adsorption characteristics significantly depend on processing variables and heat history. Further, graphite platelet orientation in the dried ink film is difficult to reproduce from sensor to sensor. These factors add great variability to the conductivity of such inks, hence cause unacceptable and undesirable variations within a product and from product to product.
Because molybdenum disulfide becomes more conductive as temperature increases, the use of molybdenum disulfide and conductive carbon black to provide the conductive paths requires changing their ratios or concentrations to adjust the conductivity of the ink for anticipated temperature conditions to be encountered. Because of the sensitivity of molybdenum disulfide to changes in temperature, compensation for temperature is difficult when the concentration of molybdenum disulfide is used by itself to adjust conductivity.
It would be desirable to provide a force transducer having improved force sensitivity, reliability, and reproduceability, as well as the additional capacity to function effectively not only at current temperatures at which force sensors are used, but at elevated temperatures, such as at temperatures of from at least 120.degree. F. to 150.degree. F. up to about 350.degree. F., while providing sufficient sensitivity and reproduceability to provide a reliable and consistent indication of applied load.